In an aircraft jet engine, air is drawn into the engine and compressed by a compressor. The compressed air is mixed with jet fuel, and the mixture is ignited and burned. The burning exhaust gases are directed against a series of turbine blades mounted on a large wheel called a turbine wheel or turbine disk, causing the turbine disk to turn. The disk is mounted on a shaft, which also supports the compressor, and the turning of the turbine disk thereby turns the compressor to maintain the continuous operation of the engine.
The materials used to manufacture turbine blades and turbine disks must be capable of operation at very high temperatures, in the neighborhood of 2000.degree. F., under high stress and fatigue loadings, and in adverse corrosive environments produced by the combustion gas. One of the primary areas of improvement of jet engines lies in raising their operating temperature to achieve higher thermodynamic efficiencies. The materials used in turbine blades and disks are pushed to the limits of their capabilities by these increases in operating temperature.
Thus, the search for higher performance, more fuel efficient jet engines is closely linked with the development of better materials for use in turbine blades and turbine disks. In its presently most significant embodiment, the current invention deals with a method of improved manufacturing of components to meet ever more demanding requirements in jet engines and other applications that require high performance from superalloys.
One of the families of metallurgical alloys is the nickel-based superalloys, which are used extensively as the materials of construction of turbine blades and disks. These alloys have excellent properties at elevated temperatures, and are presently used in most turbine blade and turbine disk applications. Although many different types have been developed, one important class of nickel-based superalloys have a number of carefully chosen alloying elements added to a nickel base. When examined under a microscope, these alloys have a structure comprising particles of Ni.sub.3 (Al,Ti), called gamma prime particles, in a matrix of grains of a nickel alloy, termed a gamma matrix.
The relative amounts and structure of the gamma prime particles and gamma matrix, the properties of these two phases, and the microstructure of the superalloy, determine the performance of the nickel-based superalloy, which in turn is the limiting factor in the ability of the turbine disk made of the alloy to operate at high temperatures. Increasing amounts of the gamma prime particles tend to give the alloy greater strength, but also make the mechanical working of the alloy to form a turbine disk more difficult. Another important consideration is the grain size of the matrix. Microstructural inhomogeneities tend to reduce the workability of the alloy, and also reduce its ability to tolerate small cracks or other imperfections in the material. Turbine disks typically fail due to small defects caused by fatigue or monotonic loadings, and it is desirable that the turbine disks be able to resist failure due to such defects when they occur. Increased grain size and reduced microstructural inhomogeneity can contribute to overall turbine disk performance.
Thus, the attainment of high performance, and the ability to fabricate high performance alloys into usable structures such as turbine disks are dual considerations in the selection of alloys and manufacturing techniques for turbine disks. In the past, turbine disks were manufactured by casting the alloy to shape, or casting and working the alloy to the final shape. One important improvement has been the development of powder metallurgical techniques for fabricating turbine disks, wherein the article is formed from a powder or particles of the superalloy, consolidated, and worked to a final shape. Furnishing the alloy as a powder or particles reduces the degree of microstructural inhomogeneity typical of prior ingot casting techniques.
However, there are several new problems which arise when working with metal powders. One significant problem relevant to the present invention is the lack of ductility in the consolidated powder preform. For a more complete discussion of this and other problems, see U.S. Pat. Nos. 3,698,962 and 3,702,791. The prior art has tried to solve this problem in several ways. Processes for filling and consolidating metal powders are well known in the art. Kasak et al, in U.S. Pat. No. 3,698,962 described a method for hot isostatic pressing powders to eliminate or significantly reduce entrapped porosity. In addition, several methods for cleaning the powder surface have been proposed by utilizing various acid washing techniques such as that described by in U.S. Pat. No. 3,704,508. Powder surface cleaning techniques using a reducing gas to clean the powder after can filling is described in U.S. Pat. No. 4,693,863 from Carpenter Technology Corp.
The foregoing processing techniques utilize an external media, either liquid or gas, to remove or enhance the powder surface to improve diffusion during consolidation. In doing so, introduction of the "cleansing" media increases the potential risk of introducing a potential reactive defect to the powder metal. Therefore in order to minimize the effects of powder contaminants, the present art for consolidating nickel superalloy powders consists of isostatically pressing the powder at a temperature below the solidus temperature or hot compacting the powder and extruding the compact before final forging to the desired configuration. The compacted powder metal material produced in such a manner is sensitive to high forging strain rates in excess of 5 in/in/min. and, therefore, is forged on specialized low strain rate equipment.
It should be apparent from the foregoing that there is a continuing need for a processing technology to fabricate high performance nickel-based superalloys into parts, such as turbine disks, with a microstructure that is conducive to achieving excellent performance in the finished part. While heretofore described in terms of turbine disks for the sake of clarity, such a process would yield important benefits in the fabrication of other parts from superalloys. The present invention fulfills this need for an improved processing technology, and further provides related advantages.